1. Field of Invention
The present invention relates generally to the field of communications systems, and particularly to the field of wireless communications, such as cellular radio.
2. Description of Related Art
Antenna diversity is a technique used in communication systems, including mobile cellular radio, to reduce the effects of multi-path distortion fading. Antenna diversity may be obtained by providing a receiver with two or more (n.gtoreq.2) antennas. These n antennas, when properly positioned, imply n channels which suffer fading in different manners. When one channel is in deep fade--that is, suffering severe amplitude and phase loss due to the destructive effects of multi-path interference, another of these channels is unlikely to be suffering from the same effect simultaneously. The redundancy provided by these independent channels enables a receiver to often avoid the detrimental effects of fading.
Alternatively, antenna diversity benefit can be provided to a mobile receiver by providing multiple transmitting antennas at a base or transmitting station, rather than at the receiver. The receiver can therefore use a single antenna, saving cost and complexity at that side of the transmission chain.
Multiple transmit antennas can be provided at the base station in a variety of ways. A schematic diagram of certain possible known techniques is illustrated in FIG. 1. Perhaps most simply, as schematically illustrated in FIG. 1(a) two antennas can be provided at the output stage, and the information signal d.sub.k can be switched between two matched antenna elements, without overlap in time or frequency. Of course this has the drawback that the transmitter requires feedback from the receiver about the channels corresponding to each transmit antenna. This scheme does not perform well when the channel is rapidly changing.
In a variant described in U.S. Pat. No. 5,479,448 and schematically illustrated in FIG. 1(b), the above mentioned drawbacks of switch diversity are removed by using a channel code to provide diversity benefit. Maximum diversity is upper-bounded by the number of antenna elements at the base station, and is equal to the minimum Hamming distance of the channel code used, provided that the receiver is equipped with one antenna. The system described in that patent is applicable to both FDD (frequency division duplex) and TDD (time division duplex)-based systems.
Illustrative embodiments of the system of U.S. Pat. No. 5,479,448 comprise a base station which employs a channel code of length n.gtoreq.2 symbols (n being the number of antennas used by the transmitter), and a minimum Hamming distance 2.ltoreq.d.sub.min .ltoreq.n. This channel code is used to encode a group of k information bits. The n antennas of the base station transmitter are separated by a few wavelengths, as is conventional to provide the diversity reception with the n antennas. The channel code symbol c.sub.i is transmitted with the i.sup.th antenna to represent these k bits. At a receiver, a conventional maximum likelihood channel code decoder provides a diversity advantage of d.sub.min.
In the preferred embodiment of U.S. Pat. No. 5,479,448, the transmitted signals from different antennas are separated in time. This results in data rate reduction, sacrificing bandwidth. The reduction in data rate is equal to the number of antennas (or length of the code).
Transmit bandwidth can be improved over the diversity arrangement of FIG. 1(b), by splitting the information signal into two paths to the two antennas, the second of which has a delay element or tap as disclosed in A. Wittneben, "Base Station Modulation Diversity for Digital SIMULCAST," 41.sup.st IEEE Vehicular Technology Society Conference Proceedings, pp. 848-853 and shown in FIG. 1(c). The signal appearing at antenna B at any given instant of time is therefore the same signal as appeared at antenna A the preceding instant of time. The two signals are transmitted simultaneously, reconstructed at the receiving station, and processed to isolate the desired information signal.